In general, a change in an individual's respiration (known as "apnea" when such a change is a transient cessation of respiration) corresponds to a change in the physical condition of that individual. In the case of invalids, infants, the elderly, hospitalized patients, or individuals with obstructive sleep apnea, a change in respiration may be attributable to present or impending physical distress. Accordingly, an urgent need exists for an improved respiratory monitoring system for such individuals that will meet technical and clinical needs for reliable monitoring of individuals at risk of respiratory distress. Many such individuals are in need of such a system, as many are unable to summon help because they may be asleep, handicapped, bedridden, hospitalized or otherwise unable to communicate. There is an especially critical need to continuously monitor some infants from birth through one year. Accordingly, the present invention relates to a pioneering, non-invasive, non-contacting, safe system which monitors the respiratory functions and/or fluctuations in exhaled breath including carbon dioxide (CO.sub.2), water vapor, various constituents in the blood gases and which determines other functions and/or fluctuations including the pH level of an individual's blood as well as the motion of the subject.
The presence of CO.sub.2 and other component gases present in exhaled breath can be detected by measuring the breath's absorption of specific wavelengths of light. With an increase in concentration of a component gas, comes a decrease in transmission of those specific wavelengths. Thus, if an emitter and a detector of infrared energy in an absorption band of a component gas are separated by an open distance, the presence (and perhaps the concentration) of gas in that open environment can be evaluated. Two basic system types can be constructed that use this principle of illumination to measure absorption characteristics: those that use a powered source of interrogation energy--called active systems; and those that use sources of energy naturally occurring in the environment--called passive systems. The passive system offers a more reliable method for monitoring a patient's respiration. A discussion of both types of systems follows:
A powered emission device and a detector comprise an active detection system. Gases passing between the emitter-detector pair will be detected and measured. Thus, it is important that the geometry of the emitter and detector be arranged to allow for the passage of a patient's breath between them. The distances can be made larger by increasing emission intensity or improving detector sensitivity (or both). The detector can be made receptive to incoming energy over a wide angle of view. Even with a wider view angle, however, only gases between the detector and the emitter will be measured. Thus, multiple emitters are required to achieve a larger detection zone volume.
Patients may assume numerous positions and orientations while being monitored. Therefore, to effect detection of CO.sub.2 in breath, when using an active system would require many emitters and detectors placed at various locations within certain proximity to the patient. Theoretically, breath, whether it travels to the side or upwards, could be detected if enough emitters and detectors were placed in enough orientations and angles. Unfortunately, some of the detectors or emitters would need to be under the patient (in the mattress if the patient is being monitored in bed for example) so as to catch breath from a patient in a side-facing position. This may not be possible from a practical point of view. Other problems with the active emitter approach include cost of the large number of emitters and detectors required to form a relative uniform region of detection, the unreliability of such a large number of components, the complicated apparatus that would be required to house such a system, and the complexity of electronics required to control the process data from numerous emitters and detectors.
In addition to the above-stated drawback, active emitters require a source of power. Photon emission is inefficient at the wavelengths of interest (i.e., 4 to 20 microns for many carbon-containing molecules). The large amounts of power needed also require a large battery to supply backup power during loss-of-power events. This also may be a substantial drawback.
In a passive system, the patient and surrounding environment become the source of interrogating energy.
Thus, all of the problems are eliminated that are associated with employing multiple emitters as discussed above. The requirements then placed on the detector assemblies are that they respond to the wavelengths emitted by the surroundings, are capable of wide-angle coverage, are sensitive enough to detect the depression in infrared energy due to presence of CO.sub.2, or other component being detected, regardless of patient orientation. The invention described herein may utilize a reference detector to cancel unwanted variation since the emitter in this system is a highly variable energy source (e.g., temperature and patient motion affect emission). A passive emission infrared gas detection system is superior to an active system in cost, reliability, and performance if these requirements are met.
Presently, devices exist on the market that monitor a patient's respiration. These devices, known as capnographs, monitor the exhaled CO.sub.2 from a patient's airways by continuously sampling the exhaled breaths. However, unlike the present invention, capnographs, require a physical attachment between the capnograph and the patient's airways. This attachment limits the clinical applications of the capnograph to intugated patients or short term monitoring applications. An even in these limited applications many difficulties arise in the monitoring process. Difficulties such as mucous plugs or other secretions blocking the sampling tube, patient movements disrupting the positioning of the sampling line, agitated patients pulling the sampling tube out of position, etc., all add to inaccuracies in the measurements taken and to the inconvenience of using the device.
Other respiratory monitoring systems have been proposed. For example, a non-contacting apnea detector is disclosed in U.S. Pat. No. 4,350,166 (Mobarry). This device monitors carbon dioxide (CO.sub.2) exhaled by an infant by detecting the difference in the infrared ("IR") radiation caused by the absorption of IR energy by exhaled CO.sub.2 of the patient. However, this device does not have the capability of accounting for changes in the level of infrared radiation caused by other factors such as the patient's general body movements, repositioning of the patient, or general disruptions in the IR energies emitted from the crib (as could be caused, for example, by a warm nursing bottle).
In U.S. Pat. No. 4,738,266, Thatcher discloses non-contact monitoring device which allegedly contains improvements over the Mobarry system. The Thatcher device contains an infrared radiating element in the system. The detection of the infant's breathing is directly related to whether the requisite amount of carbon dioxide has passed through the device-produced infrared radiation. Changes in the radiation level are registered and compared to the desired value. The infant's exhaled breath must pass through this IR source in a constant manner, or else detector values will be erroneous. This device has great clinical limitations.
Another device, which is allegedly an improvement over the Mobarry system, is disclosed in U.S. Pat. No. 4,928,703 (Wong). Wong mentions the problems associated with passive sources of radiation, such as from the patient's own motions, which are detected by the Mobarry system and which compromise the device's accuracy. The Wong device, like the Thatcher device, includes a means for emitting radiation. The radiation generated is subsequently collected and measured to determine the amount of carbon dioxide in the path of the radiation as it travels between an emitter and a collector. The only signal monitored by the Wong device is that related to the absorbed radiation which corresponds to the infant's exhaled carbon dioxide. However, this device still requires an artificial means for emitting infrared radiation, which inherently limits the number of locations at which the device can be positioned such that it operates safely and effectively. Its battery power requirements in the event of loss or unavailability of a.c. power are also excessive. Accordingly, the need still exists for a safe, reliable non-invasive, non-contacting system for monitoring an individual's respiratory condition which does not require an artificial source of infrared energy, which is capable of differentiating between infrared energy absorbed by a particular component such as carbon dioxide and other forms of infrared energy, and which may be mounted in a variety of locations without sacrificing effectiveness.